This invention relates to highly efficient processes for the preparation of tetrabromobisphenol-A.
Tetrabromobisphenol-A is one of the most widely used brominated flame retardants in the world. It is used extensively to provide flame retardency for styrenic thermoplastics and for some thermoset resins.
The commercial processes used to produce tetrabromobisphenol-A generally fall into three categories. The first category includes those processes in which substantial amounts of methyl bromide are co-produced along with the tetrabromobisphenol-A. Generally, up to 18-23 kg (40-50 pounds) of methyl bromide can be expected per 45.4 kg (100 pounds) of tetrabromobisphenol-A produced. The methyl bromide co-production is now considered desirable since there is a substantial market for this bromide as a fumigant and as a pharmaceutical or agricultural chemical intermediate. In most cases, the processes within this first category feature reacting bisphenol-A and bromine in methanol. The ar-bromination of the bisphenol-A is a substitution reaction which generates one mole of HBr per ar-bromination site. Thus, for the production of tetrabromobisphenol-A, four moles of HBr are generated per mole of tetrabromobisphenol-A produced. The HBr in turn reacts with the methanol solvent to produce the methyl bromide co-product. After the bisphenol-A and bromine feeds are finished, the reactor contents are cooked for one to two hours to complete the reaction. At the end of the reaction, water is added to the reactor contents to precipitate out the desired tetrabromobisphenol-A product.
The second category of processes features the production of tetrabromobisphenol-A without the co-production of substantial amounts of methyl bromide and without the use of oxidants to convert the HBr to Br.sub.2. See U.S. Pat. Nos. 4,990,321; 5,008,469; 5,059,726; and 5,138,103. Generally, these processes brominate the bisphenol-A at a low temperature, say 0 to 20.degree. C., in the presence of a methanol solvent and a specified amount of water. The water and low temperature attenuate the production of methyl bromide by slowing the reaction between methanol and HBr. The amount of water used, however, is not so large as to cause the precipitation of the tetrabromobisphenol-A from the reaction mass. Additional water for that purpose is added at the end of the reaction. One drawback with this type of process is that it uses a fairly long aging or cook period after the reactants have all been fed and it requires additional process time for the final precipitation of tetrabromobisphenol-A via the last water addition.
In the third category are those processes which feature the bromination of bisphenol-A with bromine in the presence of a solvent and, optionally, an oxidant, e.g., H.sub.2 O.sub.2, Cl.sub.2, etc. See U.S. Pat. Nos. 3,929,907; 4,180,684; 5,068,463 and Japanese 77034620 B4 77109/05. The solvent is generally a water immiscible halogenated organic compound. Water is used in the reaction mass to provide a two-phase system. As the bisphenol-A is brominated, the tetrabromobisphenot-A is found in the solvent. The co-produced HBr is present in the water. When used, the oxidant oxidizes the HBr to Br.sub.2, which in turn is then available to brominate more bisphenol-A and its under-brominated species. By oxidizing the HBr to Br.sub.2, only about two moles of Br.sub.2 feed are needed per mole of bisphenol-A fed to the reactor. To recover the tetrabromobisphenol-A from the solvent, the solution is cooled until tetrabromobisphenol-A precipitation occurs. This process type is not a panacea though, as there is the expense of handling, purifying and recycling the halogenated organic solvent. In addition, the cooling of the solution to recover tetrabromobisphenol-A entails additional expense and process time.
As long as there is a viable market for methyl bromide, the processes of the first category have been found to be commercially attractive. However, it is now being proposed, on an international level, that the use of methyl bromide as a fumigant be prohibited. Since the fumigant market is the main market for methyl bromide, a need is apparent for tetrabromobisphenol-A processes which do not co-produce a substantial amount of methyl bromide. This is a difficult task since such processes, to be commercially successful, will be required to economically produce tetrabromobisphenol-A without the benefit of the revenue realized from the sale of co-produced methyl bromide.
The Invention
The processes of this invention feature the efficient production of high-quality tetrabromobisphenol-A in high yields. The processes can be run in the batch mode or in the continuous mode. When run in the batch mode, process efficiency is enhanced due to relatively short reactor times as there is no need for the prior art's time-consuming post-reaction cook or aging periods or the often required post-reaction tetrabromobisphenol-A precipitation step. The use of a continuous process for the production of tetrabromobisphenol-A is unique in itself and is made possible by the short reaction and tetrabromobisphenol-A precipitation times which are features of the processes of this invention. In the continuous mode, reactor size can be substantially reduced without a loss in product output.
In addition to the above reaction efficiencies, the processes of this invention are capable of producing very pure tetrabromobisphenol-A in high yields in an alcohol-based solvent without the substantial concomitant production of alkyl bromide. Alkyl bromide yields of 5% of theory or less are possible. Even further, it is possible to obtain high yields of tetrabromobisphenol-A even though less than stoichiometric Br.sub.2 is fed to the reactor.
It has been discovered that the foregoing benefits can be obtained by (1) brominating bisphenol-A in the presence of an alcohol, e.g., methanol, and a relatively large amount of water while maintaining the reaction mass at a relatively high temperature and, (2) adding the bisphenol-A reactant to the reaction mass while the reaction mass contains from 50 to 20,000 ppm Br.sub.2. As will be discussed later, the features in (1) have heretofore conventionally been considered conducive to the low-yield production of low-quality tetrabromobisphenol-A and/or the co-production of methyl bromide.
In accordance with this invention, tetrabromobisphenol-A can be produced by a process which comprises:
a. adding bisphenol-A to a reaction mass containing (i) water and a solvent amount of an alcohol containing up to 4 carbon atoms, the water being present in an amount of from about 30 to about 85 wt %, based on the weight of the water and alcohol in the reaction mass, and (ii) at least about 50 ppm but less than about 20,000 ppm unreacted Br.sub.2 ; PA1 b. having a reaction mass temperature which is within the range of from about 45 to about 100.degree. C., and PA1 c. producing a precipitate which is at least 95 wt % tetrabromobisphenol-A and in a yield which is at least about 90%, based on the amount of bisphenol-A fed, while concomitantly producing no more than about 4.54 kg (10 lbs) of alkyl bromide/45.4 kg (100 lbs) of tetrabromobisphenol-A precipitate produced.
The brominations which occur in the processes of this invention occur with great rapidity. In fact, it is believed that the rapidity with which substantially all of the bisphenol-A is tetrabrominated is a key to obtaining the highly pure products of this invention. The reaction masses used by the processes of this invention contain significant amounts of water, thus making it incumbent that the tetrabromination occur very quickly. If it should not, the prior art teaches that the presence of large amounts of water will result in the premature precipitation of tribromobisphenol-A which will denigrate the purity sought for a tetrabromobisphenol-A product. The use of the high temperatures for the processes of this invention promote the rapid tetrabromination. However, such temperatures would normally be thought to also promote the facile production of alkyl bromide. Despite such concerns, the processes of this invention do not exhibit rampant alkyl bromide production. Instead, the processes of this invention exhibit minimum alkyl bromide production, say, from about 4.54 to 0.0454 kg (10 to 0.1 lbs) or less of alkyl bromide/45.4 kg (100 lbs) of tetrabromobisphenol-A precipitate produced, such production depending upon the process parameters. Without the use of an oxidant to convert HBr to Br.sub.2, and with methanol as the solvent, the methyl bromide production is typically found to be less than 1.8 kg (4 lbs) and preferably within the range of from about 1.8 kg (4 lbs) to about 0.454 kg (1 lb) or less/45.4 kg (100 lbs) of tetrabromobisphenol-A precipitate produced. With ethanol as the solvent, the alkyl bromide production can be less than 0.91 kg (2 lbs) and preferably within the range of from about 0.91 kg (2 lbs) to about 0.0454 kg (0.1 lb) or less/45.4 kg (100 lbs) of tetrabromobisphenol-A.
The bisphenol-A is preferably added to the reaction mass in a molten form or as a solute in a solution which comprises an alcohol solvent. Less preferred is the addition of the bisphenol-A as a solid. It is an important feature that the bisphenol-A be added to the reaction mass with its above-stated concentration of unreacted Br.sub.2. Adding the bisphenol-A in this manner is novel and is believed to contribute to the highly desired rapid tetrabromination. With the presence of the specified unreacted Br.sub.2 in the reaction mass, the bisphenol-A, as it is being fed, always has available the Br.sub.2 needed for its quick tetrabromination but not so much that degradation of the bisphenol-A structure is realized. Compare the prior art which adds the Br.sub.2 to a reaction mass of bisphenol-A. In this latter case, there is insufficient Br.sub.2 available for tetrabromination until near the end of the Br.sub.2 feed, which feed can take 0.5+ hours to effect. Even then, the prior art teaches the need for a post-curing or aging step to complete the tetrabromination. Such post-curing or aging steps entail raising the temperature of the reaction mass significantly and holding the temperature for the prescribed curing or aging period. Due to the long tetrabromination times for these prior art processes, it is not possible to use much more than about 20 wt %water in the reaction mass, as larger amounts of water will result in significant precipitation of under-brominated species, e.g., tribromobisphenol-A. Such precipitation would occur even before the Br.sub.2 feed is complete.
The maintenance of the specified concentration of unreacted Br.sub.2 in the reaction mass can be effected by either adding the Br.sub.2 to the reaction mass or by the in situ production of Br.sub.2 or by both. However the Br.sub.2 is brought to the reaction mass, its concentration can be easily adjusted to the desired levels by monitoring the reaction mass for the desired Br.sub.2 concentration and then adjusting the addition of Br.sub.2 and/or bisphenol-A to the reaction mass. If there is to be in situ production of Br.sub.3 then that production can be adjusted to compliment the adjustments to the Br.sub.2 and/or bisphenol-A additions. These monitoring and adjusting methods are more fully hereinafter described. It is preferred that the Br.sub.2 be brought to the reaction mass by simple addition. The use of the in situ Br.sub.2 production along with direct Br.sub.2 addition is also preferred.
It is preferred that the Br.sub.2 be co-fed with the bisphenol-A to the reaction mass. It is more preferred that the co-fed bisphenol-A be as a solute in a solvent comprising alcohol solvent. In a most preferred form, the solvent will also contain water. By co-feeding, it is meant that the Br.sub.2 feed period and the bisphenol-A or bisphenol-A solution feed period overlap one another to at least some extent. (A feed period is that period of time over which all of a subject feed is fed to the reactor.) For example, the Br.sub.2 feed can be to an initial alcohol/water charge followed by the bisphenol-A or bisphenol-A solution feed, with the Br.sub.2 and later feeds thereafter occurring simultaneously until finished. Another example would be that of the same initial Br.sub.2 feed followed by a continuous bisphenol-A or bisphenol-A solution feed which is accompanied by a continued, but intermittently interrupted or staged, Br.sub.2 feed. Yet, another example is that of initiating the Br.sub.2 feed to an alcohol and water pre-charged reactor followed by the bisphenol-A or bisphenol-A solution feed so that the two feeds occur simultaneously until the specified amount of Br.sub.2 has been fed. At that point, the bisphenol-A or bisphenol-A solution feed continues alone until it is finished. Also, the Br.sub.2 and bisphenol-A or bisphenol-A solution feeds can be, timewise, completely concurrent one with the other. Other co-feeding schemes are possible, it being required that the scheme must more closely approximate the addition of the bisphenol-A to a reaction mass having the before-specified unreacted Br.sub.2 concentrations than it approximates the addition of Br.sub.2 to a bisphenol-A reaction mass.
Commercially available Br.sub.2 is suitable for use as the Br.sub.2 feed. Should the Br.sub.2 contain undesirable impurities, it can be treated by conventional purification techniques, e.g., distillation, H.sub.2 SO.sub.4 treatment, etc., which are well known to those skilled in the art.
The Br.sub.2 can be fed as a liquid or as a gas to the reactor. It is preferred that the feed be gaseous. Whether the Br.sub.2 is liquid or gaseous, it is preferred that the feed entry point be sub-surface of the reaction mass. This is conveniently accomplished by use of a dip tube in a reaction vessel. If the feed is liquid, above-surface feed must contend with possible splattering and inefficient mixing.
Processes of this invention feature the use of water in the reaction mass which is within the range of from about 30 to about 85 wt %, based upon the total amount of water and alcohol solvent in the reaction mass. Preferably, the amount of water in the reaction mass is within the range of from about 30 to about 75 wt % water. Most highly preferred is the range of from about 30 to about 70 wt %. When the alcohol solvent is methanol, the preferred amount of water is from about 30 wt % to about 55 wt %. Should the alcohol solvent be ethanol, then the preferred amount of water is from about 40 wt % to about 65 wt %.
The water content of the reaction mass is an important aspect of this invention. As before stated, it is believed, for the processes of this invention, that the water content greatly attenuates the formation of methyl bromide while, unexpectedly, allowing for a high yield of a high purity tetrabromobisphenol-A product. It is theorized, though the processes of this invention are not to be limited by any theory, that the formation of methyl bromide is attenuated because the HBr, which is co-produced by the substitution bromination reaction between the aromatic moieties of bisphenol-A and Br.sub.2, is diluted by the large amount of water in the reaction mass. Further, the HBr reacts with the water to yield H.sub.3 OBr which is very slow to react with the alcohol in the reaction mass. As pointed out previously, the attenuated alkyl bromide formation for the processes of this invention does not generally exceed about 4.54 kg (10 lbs) of alkyl bromide/45.4 kg (100 lbs) of tetrabromobisphenol-A precipitate produced.
With regard to the high yield of a highly pure tetrabromobisphenol-A product, it is noted that typical products from the processes of this invention have a tetrabrombisphenol-A purity of at least 95 wt %, based on the total weight of the recovered product, and a tetrabromobisphenol-A yield of at least 90%, based on the bisphenol-A fed. This yield and purity are believed to be due to the large concentration of water in the reaction mass. Without being limited to any particular theory, it is believed that the water enhances the presence of brominating species in the reaction mass. With this enhancement, there is a favoring of the rapid bromination of the bisphenol-A all of the way to tetrabromobisphenol-A, all before the intermediate, tribromobisphenol-A, has sufficient opportunity to form a significant amount of precipitate. It is believed that the enhancement of the brominating species is due to the fact that, as before stated, HBr reacts with water to form the H.sub.3 OBr acid. The H.sub.3 OBr acid does not react as readily with Br.sub.2 as does HBr. This is important because, if H.sub.3 OBr was not formed, a larger quantity of HBr would be available to react with Br.sub.2 to form HBr.sub.3. The HBr.sub.3 is not desired as it is a weak brominating species in the reaction mass and its formation consumes Br.sub.2. With less Br.sub.2 available, there is a slowing of the bromination reaction. This slowing can result in an increase in the precipitation of tribromobisphenol-A.
The water in the reaction mass can be supplied thereto by simply being fed to the reactor as a direct water feed or it may be supplied as a constituent of the bisphenol-A/alcohol solvent solution or it may be supplied via a direct water feed and as a constituent of the bisphenol-A/alcohol solvent solution. Supplying the water as part of such a solution is convenient and preferred. If the water is introduced to the reaction mass as a separate feed stream, then it may or may not be fed essentially contemporaneous with the feed of the bisphenol-A/alcohol solvent solution. Even further, a portion, if not all, of the water can be fed as steam or steam condensate along with a gaseous Br.sub.2 feed. The stem could have been used to vaporize the Br.sub.2 to form the gaseous feed. Another example features supplying water as a charge or as part of a charge to the reactor prior to initiating the feeds and adjusting the amount of water later fed to obtain the desired water content in the reaction mass. However the water is supplied to the reaction mass, it is preferred it be such that the proper amount of water be present in the reaction mass during substantially all of the bisphenol-A feed.
In those cases where the amount of water used is in the lower end of the range, say 30 to 35 wt %, and the other process parameters have not been optimally selected, it may be desirable to add some additional water at the end of the bisphenol-A feed. The possible advantage to such an addition is that the additional water may cause further precipitation of tetrabromobisphenol-A from the reaction mass. The further precipitation goes towards increasing the yield of the process.
The alcohol solvent can be supplied as an individual feed or as a solvent constituent of the bisphenol-A solution feed or as both From a practical standpoint though, the alcohol solvent is best fed as a solution constituent. The amount of alcohol solvent in the reaction mass is that amount which will insure, in the presence of the water, that the bisphenol-A and its under-brominated intermediates, i.e., mono-, di- and tri- bromobisphenol-A, are essentially soluble and that the desired tetrabromobisphenol-A is highly insoluble. Such amounts are referred to herein as a "solvent quantity". Generally, the amount of alcohol solvent used, expressed as a weight ratio of alcohol to bisphenol-A fed, is within the range of from about 1.3:1 to about 10:1. Preferred for methanol is the range of from about 2 to about 5. For ethanol, the preferred range is from about 1.5 to about 4. A most preferred range for methanol is from about 2.5 to about 4. If the bisphenol-A is to be supplied as a solution in the alcohol solvent, then the alcohol solvent can be supplied to the reaction mass via the solution feed or as a part of the solution feed and the remainder via a separate alcohol feed. The amount of alcohol solvent in the solution is, at a minimum, that amount which will at least provide a flowable slurry and, preferably, a free-flowing liquid. The practitioner can empirically determine the minimum amount needed for the particular alcohol and feed temperature chosen.
When choosing the weight ratio of alcohol to bisphenol-A fed, it should be noted that the lower ratios, say 1.3 to 2, can result in a high concentration of HBr in the reaction mass. Reaction mass concentrations of HBr much in excess of 35 wt % are generally to be avoided. It is believed that HBr concentrations of 35+wt %, especially 40+wt %, can cause degradation of and/or inferior product. So that the practitioner can use the lower alcohol to bisphenol-A weight ratios, it is desirable to attenuate the HBr concentration in the reaction mass by feeding an oxidant to the reaction mass, e.g., H.sub.2 O.sub.2 or Cl.sub.2, to oxidize the HBr to Br.sub.2. In this way, the HBr concentration is attenuated and Br.sub.2 is made available to the reaction mass, this latter effect reducing the amount of Br.sub.2 that needs to be fed to the reactor.
In a most preferred form, the bisphenol-A is fed as a solute in solution with an alcohol solvent and water. The most highly preferred mode of operation is to supply essentially all of the bisphenol-A, alcohol solvent and water to the reaction mass via such a solution (Some water can be introduced to the reaction mass as the result of the formation of alkyl bromide and in those cases where H.sub.2 O.sub.2 is used to oxidize HBr to Br.sub.2. This water goes towards the total water in the reaction mass.) Such a preferred mode simplifies insuring the proper reaction mass composition. Indeed, it is most preferred that the bisphenol-A/alcohol solvent/water solution mimic the alcohol solvent/water composition of the reaction mass. Thus, a preferred solution will contain from about 30 to about 85 wt % water, based upon the weight of the alcohol solvent and water. Other preferred ranges mimic the preferred ranges for the reaction mass.
The alcohol solvent is a C.sub.1 to a C.sub.4 alcohol which, in the prescribed amount, is capable of dissolving the Br.sub.2, bisphenol-A, monobromobisphenol-A, dibromobisphenol-A and tribromobisphenol-A under reaction conditions. The reaction conditions of special import are the reaction mass temperature, the presence of unreacted Br.sub.2 in the reaction mass and the reaction mass water content. Further, the alcohol should be substantially inert with regard to H.sub.3 OBr and the ar-bromination of the bisphenol-A to tetrabromobisphenol-A. The alcohol also should not contribute to the production of troublesome amounts of color bodies, ionic bromides and/or hydrolyzable bromides. Hydrolyzable bromides include 1-bromo-2-methoxy-2-(3',5'-dibromo-4'-hydroxyphenyl)propane, 1,1-dibromo-2-methoxy-2-(3',5'-dibromo4'-hydroxyphenyl)propane, 1,3-dibromo-2-methoxy-2-(3',5'-dibromo-4'-hydroxyphenyl)propane, and 1,1,3-tribromo-2-methoxy-2-(3',5'-dibromo4'-hydroxyphenyl)propane. The alcohol, when taken in combination with the water and reaction conditions of the processes of this invention, can have some small ability to dissolve tetrabromobisphenol-A, but for the sake of reaction yield, the dissolving power should be low, say no more than about 10 wt % and preferably no more than about 5 wt % dissolved tetrabromobisphenol-A in the liquid phase of the reaction mass, the weight percent being based on the weight of the liquid phase.
Exemplary of the preferred alcohol solvents are methanol, ethanol propanol, butanol and mixtures of any two or more of the foregoing. Polyhydric alcohols such as ethylene glycol and glycerine and any mix of the foregoing are also suitable. Further, any mix of any of the alcohols of this invention is suggested. Most preferred are methanol, ethanol and propanol, with methanol and ethanol being the solvents of choice. Methanol and ethanol are relatively inexpensive and are easily recovered by simple distillation techniques for recycle. Since there is a large water presence in the processes of this invention, it is not necessary to recover the alcohol with a low water content, thus reducing the alcohol recovery cost.
When choosing the alcohol to be used, the practitioner should consider that the alcohol chosen will determine the alkyl bromide produced. Hence, methanol will yield methyl bromide, and ethanol will yield ethyl bromide. Even though the amount of alkyl bromide produced by the processes of this invention is small, there still may be some preference for the alkyl bromide produced and that preference might be addressed by the alcohol chosen.
The reaction mass is essentially a two-phase system, a liquid phase and a solid phase. The former will comprise the reactants, by-product HBr, the alcohol solvent and the water, while the latter will comprise the tetrabromobisphenol-A precipitate. The reaction mass is absent of any need for a second liquid phase, such as that which could be provided by a water immiscible organic compound, see U.S. Pat. No. 3,929,907. The volume of the liquid phase is generally defined by the amount of alcohol solvent and water present, the other constituents only making a minor contribution. The liquid phase volume should be consistent with providing a stirrable and manageable reaction mass but not so large that the volume burdens the process with a need for an overly large reactor and a materials handling problem.
The feed streams are preferably at a temperature which promotes efficient obtainment of the desired reaction mass temperature. A suitable liquid Br.sub.2 feed temperature is from about 10.degree. C. to just below the boiling point of Br.sub.2. If the Br.sub.2 is to be fed as a gas, then the Br.sub.2 stream temperature should be that which is conducive to such a feed. For example, such a feed temperature may be within the range of from about 60 to about 100.degree. C. The bisphenol-A/alcohol solvent solution and/or the individual feed temperatures should be that which do not detrimentally cool or heat the reaction mass and which allow for the feeds to be made in the liquid state.
The Br.sub.2 and bisphenol-A/alcohol solvent solution and/or separate feeds all contribute to the formation of the reaction mass in the reactor. A portion of the Br.sub.2 fed to the reactor and/or produced in situ will be consumed in the bromination reaction. The non-consumed Br.sub.2 preferably provides, until it in turn is consumed, for the before-described excess of unreacted Br.sub.2 in the liquid phase.
As a statement of general principle, the presence of the unreacted Br.sub.2 in the reaction mass is for the purpose of keeping the Br.sub.2 content in the reaction mass ahead of the bisphenol-A feed, thus assuring a perbromination condition without unwanted side effects. The presence of unreacted Br.sub.2 in the liquid phase is extant as bisphenol-A is being fed. It is preferred that the unreacted Br.sub.2 presence be coextensive with the bisphenol-A feed. It is permissible, however, for the unreacted Br2 content to stray out of the before-specified ranges for brief periods of time depending on the level of under-brominated species that can be tolerated in the tetrabromobisphenol-A reaction product and/or upon the extent of precipitation of the under-brominated species which is realized. In fact, if the period of time is very brief and favorable reaction parameters are chosen, the formation of these under-brominated precipitates may not occur to any appreciable extent at all. The practitioner will have to observe the process and determine by empirical methods the sensitivity of the chosen reaction conditions to the brief straying of unreacted Br.sub.2 content from the specified ranges. Thus, for the purposes of this invention the "presence of unreacted Br.sub.2 " can encompass brief periods of time in which the unreacted Br.sub.2 content is not within specification, but which does not result in the formation of under-brominated species to an extent that results in an unacceptable tetrabromobisphenol-A product, say one containing less that about 95 wt % tetrabromobisphenol-A.
Quantifying the preferred amount of unreacted Br.sub.2 in the reaction mass liquid phase is best handled by a trial and error technique. A trial process is first defined by choosing an unreacted Br.sub.2 level and the other process parameters. The produced tetrabromobisphenol-A product from the process is analyzed for its tri- and tetrabromobisphenol-A content. If the tribromobisphenol-A level is too high another trial process is constructed with a higher unreacted Br.sub.2 level or with different process parameters, say raising the reaction mass temperature or increasing the residence time. The procedure is repeated until the desired product is obtained. Caution should be taken when adjusting the process parameters to insure that the production of unwanted by-products, e.g., tribromophenol, does not become a problem. Once the desired product has been obtained, the unreacted Br.sub.2 content used is then measured.
Measuring the unreacted Br.sub.2 content can be performed by the use of colorimetric techniques. One technique comprises first forming an acidic (HBr) water and methanol solution. From this solution, several equivolume samples are drawn. To each sample is added a different and measured amount of Br.sub.2. The colors of these sample solutions are then compared colorimetrically with the color of the reaction mass liquid phase used in the test process. A color match indicates that the Br.sub.2 content in the liquid phase of the reaction mass is equal to that in the sample. Colorimetric determination for unreacted Br.sub.2 is suitable as color correlates nicely with unreacted Br.sub.2 content. Low concentrations give a pale yellow color, intermediate concentrations give a strong yellow color, high concentrations give an orange color, and the highest concentrations give a dark red color. For the wide range of process conditions disclosed herein, unreacted Br.sub.2 concentrations in excess of 50 ppm, but less than about 20,000 ppm, are to be considered. Preferably, the unreacted Br.sub.2 content will be within the range of from about 50 to about 15,000 ppm, and most preferably within the range of from about 2,000 to 6,000 ppm. The ppm values are based upon the weight of the reaction mass liquid phase (liquid portion).
Once the process parameters have been chosen, the maintenance of the unreacted Br.sub.2 concentration in the reaction mass can be accomplished by measuring, during the bisphenol-A feed, the Br.sub.2 concentration, e.g., by the above-described colorimetric technique, then adjusting the Br.sub.2 feed, the bisphenol-A feed or both to obtain the desired Br.sub.2 concentration as again determined by the colorimetric technique. Since there will be tetrabromobisphenol-A precipitate in the reaction mass, colorimetric monitoring may require that a small stream be taken from the reactor and filtered to remove the solids before being submitted to a colorimetric technique. If removal and filtration is difficult, the reaction mass color may be read by the use of reflectance techniques which measure the intensity of the light reflected off of the reaction mass. Also, color comparisons can be made by use of a portable Hunter Mini-Scan/EX colorimeter and by comparing the Hunter "a" values of the samples and the reaction mass. The "a" values appear to correlate well with the excess Br.sub.2 content The Mini-Scan/EX colorimeter operates by producing a flash of white light onto a sample. The reflected light is dispersed by a holographic grating onto a diode-array detector. Percent reflectance data is obtained every 10 nm from 400 to 680 nm. This data is used to calculate the L, a, b, and YI values. In all of the colorimetric cases, the color of the liquid phase of the reaction mass is the determinative factor.
It is to be understood that techniques other than colorimetric techniques may be used in monitoring to obtain the desired unreacted Br.sub.2 level in the reaction mass. Though the particular technique used is not critical to the processes of this invention, the use of the colorimetric technique is highly preferred.
It is also to be understood that the method used to obtain the desired unreacted Br.sub.2 level can be by a method other than the adjustment of the before-mentioned feeds. For example, when an oxidant is used to convert Br to Br2, the amount of Br.sub.2 generated can be regulated by controlling the amount of oxidant fed to the reaction mass. The amount of unreacted Br.sub.2 contributed to the reaction mass by oxidation of HBr can be substantial considering that four moles of HBr are generated for each mole of tetrabromobisphenol-A produced. Thus, when additional Br.sub.2 is needed, the practitioner can use the oxidation of HBr to generate at least a part of the Br.sub.2 needed to obtain the desired unreacted Br.sub.2 level.
With the use of an oxidant to oxidize the HBr to Br.sub.2, the processes of this invention can obtain good results by feeding only about two moles or slightly more of Br.sub.2 to the reactor for every one mole of bisphenol-A fed. (When speaking of the amount of Br.sub.2 fed per mole of bisphenol-A, the context is the relationship between the overall total amounts of these two feeds and is not meant to describe the relationship at any one time in the reaction mass.) The other two moles of Br.sub.2 that are needed can be provided by the full oxidation of the co-generated HBr. If there is less than full HBr oxidation, then the total amount of Br.sub.2 fed to the reactor will be that amount, in sum with the Br.sub.2 formed by oxidation, which will provide at least stoichiometric quantities of Br.sub.2 and, preferably, quantities which are in slight excess of stoichiometric, say from about 0.1% to about 3% percent of stoichiometric. Stoichiometric Br.sub.2 for the ar-tetrabromination of bisphenol-A is four moles of Br.sub.2 per mole of bisphenol-A. As can be appreciated, if the oxidation of HBr is not part of process, then the Br.sub.2 feed would be at least stoichiometric (four moles of Br.sub.2 per mole of bisphenol-A fed), with the slight excess being preferred, e.g., up to about 4.1-4.25 moles of Br.sub.2 per mole of bisphenol-A fed.
For batch processes, the excess Br.sub.2 present after completion of the process can be removed by treating the reaction mass with a reducing agent, such as sodium sulfite or hydrazine, or by slowly feeding excess bisphenol-A until essentially all bromine has reacted.
If an oxidant is used to convert HBr to Br.sub.2, the oxidant can be any which is capable of oxidizing HBr to Br.sub.2 in the reaction mass and under the process conditions of this invention, all without deleterious affect on the production of tetrabromobisphenol-A in high purity and in high yields. Preferred oxidants are those in liquid or gas form which facilitates their feed to the reactor. Preferred oxidants are chlorine and hydrogen peroxide.
When Cl.sub.2 is the oxidant, it can be fed to the reaction mass as a gas or as a liquid. The gaseous feed is preferred. To mitigate against the formation of chlorinated bisphenol-A, it is preferred that the Cl.sub.2 be fed after initiation of the Br.sub.2 feed. After the initial Br.sub.2 feed, Cl.sub.2 can be fed contemporaneously with the Br.sub.2 feed. Even with this feed sequence, some bromochlorobisphenol-A compounds will be formed. Fortunately, these bromochloro species are present in very minor amounts, say from about 50 to about 500 ppm, based on the total weight of the precipitate. The most predominate bromochloro specie will, in most cases, be chlorotribromobisphenol-A.
When the oxidant is H.sub.2 O.sub.2, safety makes it preferable that it be fed to the reaction mass in an aqueous solution containing no more than about 90 wt % H.sub.2 O.sub.2. Preferred are aqueous solutions containing from about 30 to about 80 wt % H.sub.2 O.sub.2. A most preferred solution is one containing from about 50 to about 70 wt % H.sub.2 O.sub.2.
The H.sub.2 O.sub.2 can be fed to the reaction mass at any time. For batch operation, it is preferred that the H.sub.2 O.sub.2 be fed after most of the Br.sub.2, say above about 50%, has been fed. For continuous operation, the H.sub.2 O.sub.2 feed would most preferably occur contemporaneously with at least most of the Br.sub.2 feed. Most preferably, the H.sub.2 O.sub.2 feed would start after initiating the Br.sub.2 feed.
The oxidants can be fed to the reaction mass separately or in some cases, along with the Br.sub.2 feed. It is preferred that the Cl.sub.2 be fed through the same feed conduit as is the Br.sub.2 and may be fed while Br.sub.2 is being fed. In distinction, the H.sub.2 O.sub.2 is preferably fed as a separate feed stream
The amount of oxidant fed is preferably that amount needed to oxidize that amount of HBr which is needed to yield the desired amount of Br.sub.2. Both H.sub.2 O.sub.2 and Cl.sub.2 are capable of oxidizing HBr on a one mole to two mole basis. Thus, relating the oxidant to the bisphenol-A feed and depending upon the amount of HBr that is set for oxidation, a typical mole ratio of H.sub.2 O.sub.2 or Cl.sub.2 to bisphenol-A will be within the range of from about 1:1 to about 2:1. A more preferred mole ratio is from about 1.5:1 to about 1.9:1. The higher oxidant ratios are preferred when H.sub.2 O.sub.2 is the oxidant, while the mid-range ratios, say 1.5-1.8:1, are preferred when Cl.sub.2 is the oxidant. The reason that the lower oxidant ratios are preferred for Cl.sub.2 is that there is a balance between the amount of HBr oxidized and the amount of chlorobromo species which can be tolerated. If there is no need to keep the chlorobromo species to some minimum amount, then more Cl.sub.2 is permissible. Adjustments to the above ranges are necessary if the oxidant chosen does not oxidize the HBr on a one to two basis. In these cases, the ranges are adjusted in proportion to the variance in the one to two relationship.
Another important consideration in practicing the processes of this invention is the reaction mass temperature. It is desirable to use a relatively high temperature so that the bromination of the bisphenol-A to tetrabromobisphenol-A will be sufficiently fast to attenuate the formation of tribromobisphenol-A precipitate. However, there is a practical limit as to how high the temperature can be. For example, the practitioner would not want to use temperatures which would cause the production of unacceptable levels of unwanted by-products or the degradation of the tetrabromobisphenol-A product.
It is unusual to operate a tetrabromobisphenol-A process at relatively high temperatures during the addition of a reactant, that is, for the instant case, bisphenol-A. This is especially so when the production of alkyl bromide is to be minimized. It is conventional to expect that high temperatures will yield large amounts of methyl bromide. Also, the use of high temperatures is not conventional when the precipitation of the tetrabromobisphenol-A is to occur under reaction conditions soon after it is formed-such precipitation being a feature of the processes of this invention. It would be expected that high temperatures would frustrate such precipitation by increasing the solubility of the tetrabromobisphenol-A in the solvent solution and require a final cooling of or addition of water to the reaction mass to effect the desired precipitation. The processes of this invention are not so affected, nor is there required a cooling step to obtain tetrabromobisphenol-A precipitation. In addition, the use of the higher temperatures of this invention reduce process costs as the processes can use cooling tower water to cool the reactor instead of having to use refrigeration which is required by the low temperature processes.
Preferred reaction mass temperatures are within the range of from about 30 to about 100.degree. C. More highly preferred temperatures are within the range of from about 50 to about 100.degree. C. The most highly preferred temperatures are within the range of from about 50 to about 75.degree. C. Temperatures of 30 to 50.degree. C. are best when the liquid phase of the reaction mass is large.
The bromination of bisphenol-A is an exothermic reaction as is the oxidation of HBr with an oxidant. To control the reaction mass temperature, it may become necessary to remove heat from the reaction mass. Heat removal can be effected by running the reaction at reflux with a condenser facilitating the heat removal. If it is desired to operate at a temperature below the atmospheric boiling point of the reaction mixture, the reaction can be run under sub-atmospheric pressure.
Generally, the basic concepts of the processes of this invention are not appreciably affected by the process pressure. Thus, the process can be run under sub-atmospheric, atmospheric or super-atmospheric pressure.
At process initiation, it is desirable to charge the reactor with a liquid pre-reaction charge which will become a part of the reaction mass upon the commencement of the feed. The liquid charge will provide a stirrable reaction mass and act as a heat sink to moderate temperature changes in the reaction mass. The liquid charge is preferably comprised of water and the alcohol solvent fed in the solution. It is preferred that the liquid charge be acidic, e.g., containing from 1 to 30 wt % acid such as a hydrohalogenic acid, e.g., HBr, HCl, or the like. The acid seems to promote good color in the initial tetrabromobisphenol-A produced. Further, it is preferred that the solvent be saturated with dissolved tetrabromobisphenol-A. It is also preferred that the reactor be charged with seed particles of tetrabromobisphenol-A. The saturation of the solvent and the presence of the seed particles both act to enhance the precipitation of the tetrabromobisphenol-A produced during the bromination period. It is most practical to use a heel from a previously run process of this invention as the liquid charge. The tetrabromobisphenol-A seed particles can be brought over from the previous run or can be added separately. If a heel is not available, it is also possible to use a separate pre-reaction charge of water, acid, and alcohol solvent. The only caveat to this scheme is that there must be apportionment of the various feeds so that there will still be compliance with the various parameters which define the processes of this invention.
It is beneficial to insure that the reaction mass liquid portion is acidic in nature. This can be easily accomplished by insuring a presence of HBr in the reaction mass during at least a portion of the reaction period, and preferably during all of the reaction period. If the HBr produced from the aromatic bromination is not oxidized to Br.sub.2, then that HBr can act to accomplish the acidification. Generally speaking, the reaction mass liquid portion should contain from about 1 wt % to about 30 wt % HBr, based on the total weight of liquid portion of the reaction mass. Most preferred is the range of 7 wt % to about 20 wt % HBr. It is most preferred to have an acidified reaction mass at process initiation. This can be accomplished by using the before-mentioned "heel" or acid/alcohol solvent/water pre-charge. Having an initial acidic condition is more important when the process is run in the batch mode as the product produced without such a condition becomes part of the total product produced. In the continuous mode, the product produced without the initial acid condition will be produced during the first hours of process start-up. This sub-standard product can then be recovered and discarded. As the continuous process runs further, the acid build-up in the reaction mass reaches steady state and becomes sufficient. Product produced under these acid conditions can then be recovered without contamination from the initially produced product. In the case of using an oxidant to convert the HBr to Br.sub.2, care must be taken to not oxidize all of the HBr in the reaction mass and to thus leave sufficient HBr present to give the acid condition and acquire the color benefit. While HBr is the preferred acid, other mineral acids may be used, HCl, HF, HI or mixtures thereof
When the process of this invention is run as a batch process, the Br.sub.2 and bisphenol-A feeds are fed to a stirred reactor until they are exhausted. There is no need for a post-feed cook or aging period of any significant length as, under the reaction conditions, the bromination of bisphenol-A to tetrabromobisphenol-A occurs quite rapidly. Also, since the water content of the reaction mass is so large and since the tetrabromobisphenol-A is so insoluble in the presence of such an amount of water, there is generally no need for or, at best, only a modicum of benefit is obtained by cooling the final reaction mass to obtain further precipitation. The benefit of cooling resides mainly in reducing the vapor pressure of solvated gaseous bromides, e.g., methyl bromide, in the reaction mass prior to the liquid-solids separation. There also could be some slowing of the formation of these bromides. Finally, depending on the separation technique used, cooling the reaction mass may make it easier to handle downstream from the reactor. Thus, if none of the above are of concern or relative value, then the reaction mass can be simply subjected to liquid-solids separation as soon as the bisphenol-A feed is finished. From a practical standpoint though, some time will lapse as the reaction mass will need to be transported to the separation equipment. If, however, cooling is desired, the cooling time will depend upon how the reaction mass is to be cooled and to what temperature it is to be cooled. In a laboratory setting, cooling times can range from about one to about thirty minutes.
After the recovery of the solids from the liquid, the solids are preferably washed with a solution of water and the particular alcohol used in the reaction. The washing removes essentially all the mother liquor from the solids. The mother liquor contains impurities such as tribromophenol HBr, and hydrolyzable impurities. A typical wash can be a 30 wt % methanol or ethanol in water solution. The washed solids are then rewashed with deionized water to remove any remaining water miscible solvent from the first wash so as to minimize emission problems when drying the product.
When run in the continuous mode, the reactor is preferably a continuously stirred tank reactor. The reaction mass is being continuously formed and a portion thereof is being removed from the reactor during the reaction mass formation. The reactor design should be such that the average residence time in the reactor is sufficient to insure the tetrabromination of substantially all of the bisphenol-A. The feeds to the reactor and the precipitate removals can be interrupted so long as they are recurrent. The terms "continuous feed" and "continuous withdrawal" mean being characterized by continued occurrence or recurrence and are not meant to exclude interrupted feeds or withdrawals. Generally, such interruptions are of short duration and may be suitable depending upon the scale and design of the reactor. For example, since the tetrabromobisphenol-A precipitate will tend to settle near the bottom of the reactor, a withdrawal may be made and then stopped for a period of time to allow for precipitate build-up to occur prior to the next withdrawal. Such a withdrawal is to be considered continuous in the sense that the withdrawal does not await the completion of the reactor feeds and is recurrent. Such features are not generally thought of as being characteristic of batch processes.
Whether the continuous withdrawal is interrupted or not, the withdrawal results in a portion of the liquid and a portion of the solids in the reaction mass to be withdrawn together. The solids portion will be predominately tetrabromobisphenol-A. The solids portion can be filtered, the precipitate washed, etc., as is done for the above-described batch mode case.
When using the continuous mode of operation, it is believed that it would be beneficial if the reaction mass temperature be kept fairly high as compared to the temperatures preferred for the batch mode. Preferred batch mode temperatures are from about 50 to about 65.degree. C. For the continuous mode, the preferred temperatures are within the range of from about 55 to about 95.degree. C., and most preferably within the range of from about 65 to about 95.degree. C. Very good results are predicted with temperatures of from about 65 to about 75.degree. C. By using the higher temperatures, it was found that higher purity product could be obtained.
The benefit of high temperatures on product purity is understood in view of studies which support the correlation between product purity and the relative rates of bromination and precipitation of the tribromobisphenol-A intermediate. Raising the temperature benefits both the reaction rate and the solubility of the tribromobisphenol-A in the reaction mass liquid phase and thus, promotes the obtainment of a pure product. An increase in Br.sub.2 or an increase in the tribromobisphenol-A concentration in the liquid phase by reducing the liquid phase alcohol and water content can also increase the bromination rate of the tribromobisphenol-A, but, both present problems of their own A high Br.sub.2 concentration can cause the formation of undesirable by-products, while decreasing the liquid phase alcohol and water content will increase the HBr content of the reaction mass. The result is a reduction in the tetrabromobisphenol-A product purity.
It is acted that in the continuous mode of operation, the preferred reactor residence time should be within the range of from about 10 to about 150 minutes when using a continuously stirred tank reactor and the process conditions which are preferred for that operating mode. More preferred residence times are within the range of from about 15 to about 90 minutes. Most preferred is from about 20 to 70 minutes. Reactor residence time, as used herein, is the volume of the reactor contents divided by the flow rate at which slurry is removed from the reactor.
The tetrabromobisphenol-A product produced by the processes of this invention contains at least about 95 wt % tetrabromobisphenol-A and preferably at least about 97.5 wt % tetrabromobisphenol-A and, most preferably, at least about 98.5 wt %. The best products are those having above about 99 wt % tetrabromobisphenol-A. All wt % are based on total weight of the precipitate. The product quality is excellent, having an APHA color less than about 50 (80 grams of tetrabromobisphenol-A in 100 ml of acetone). Preferably, the APHA color range is between 25 and 50. Hydrolyzable bromides are also kept low, generally below about 60 ppm. The process yields are impressive, with yields being at least about 90% and preferably within the range of from about 95 to about 99%.
As can be appreciated from the foregoing, the bisphenol-A feed, the water content of the solvent, the reaction temperature and the Br.sub.2 content in the reaction mass during the bisphenol-A feed all contribute to obtaining the desired tetrabromobisphenol-A product in an efficient manner. The selection of particular values for each of these process parameters to obtain the results desired will depend on each practitioner's needs and upon the equipment available. One practitioner may emphasize one benefit of using a process of this invention over other possible benefits. Thus, that practitioner may select different process parameter values than those selected by another practitioner who desires to highlight other benefit(s).
A preferred set of process parameters is: alcohol/BPA wt ratio of 1.3-5; Br.sub.2 content in the reaction mass--about 2,000 to 6,000 ppm; process temperature--50-75.degree. C.; residence time (continuous mode)--about 30 to 70 minutes; and weight percent H.sub.2 O--about 35 to 60 wt %, based on weight of H.sub.2 O and alcohol solvent. Most preferred is the set of: alcohol/BPA wt ratio of 2-4; Br.sub.2 content in the reaction mass--3,000 to 6,000; process temperature--about 60-75.degree. C.; residence time (continuous mode) 40-50 minutes; and weight percent H.sub.2 O--about 40-55 wt %, based upon weight of H.sub.2 O and alcohol solvent.
The use of oxidation to generate Br.sub.2 is particularly attractive in those cases where the oxidation is more economical than the cost of providing for an equivalent amount of Br.sub.2 in the feed to the reactor. The economic advantage is usually extant in those cases where the costs of feeding four moles of Br.sub.2 minus the value of recovered HBr is greater than the costs of feeding two moles of Br.sub.2 plus the oxidation of the HBr.
Though preferably designed to minimize the production of methyl bromide, the processes of this invention are sufficiently adaptable to be modified to produce moderate amounts of alkyl bromide. Alkyl bromide production can be increased by cooking or aging the reaction mass post the tetrabromobisphenol-A formation to give sufficient time for the alcohol-HBr reaction to occur. For the continuous mode of operation, the residence time would be increased beyond that which is needed to produce the tetrabromobisphenol-A precipitate.
While the foregoing descriptions of the oxidation of HBr generally speak of the HBr being oxidized in the reactor or reaction mass, it is within the scope of the processes of this invention to also remove co-produced HBr from the reactor and oxidize it outside of the reactor and to then send the so produced Br.sub.2 back to the reactor.
It is also within the scope of the processes of this invention to provide HBr to the reactor from a source other than the reaction in the reactor. This non-indigenous HBr can be oxidized along with the co-generated HBr to yield Br.sub.2. The Br.sub.2 produced from the non-indigenous HBr can then count against the total Br.sub.2 needs of the process and the appropriate adjustment in the Br.sub.2 feed can be made. The non-indigenous HBr feed can also be adjusted to insure an acidic reaction medium despite the oxidation of HBr.